Aye, you must subscribe to keep playing.
One thing you might want to consider would be to talk to your father about your computer's capabilities and the game's suggested specs. It is possible that your system isn't up to running Horizons; if that's the case, it would be better to know before trying to do the download.
You could also try to get your father interested in playing. [:D] On the good side, it would almost guarentee a hardware upgrade. The downside is that you'd probably get less playing time if he really likes the game. [;)]